It is known to use a catalytic converter in the exhaust gas flow path of a motor vehicle with an internal combustion engine. The catalytic converter includes a substrate with a coating of a catalyst material that catalyzes the oxidation of hydrocarbon molecules and carbon monoxide molecules as well as the reduction of nitrogen oxides emitted in the vehicle exhaust gas. The catalyst performs efficiently when the catalyst is above a minimum temperature to stimulate the desired oxidation and the exhaust gas has the proper fuel to air ratio (F/A).
It is desirable to optimize the fuel to air ratio delivered to the engine for performance as well as emissions control. Emissions control is increasingly difficult at startup because a higher fraction of delivered fuel remains unburned and is subsequently exhausted, and the catalyst has not reached its operating temperature. Methods have been employed to reduce exhaust emissions and increase catalytic converter performance including increasing engine speed at idle, retarding ignition timing and/or reducing delivered fuel. These methods however have been known to have drawbacks. For instance, increasing idle speed reduces fuel economy and may increase engine noise and vibration. In addition, ignition retard and fuel reduction may cause a rough idle.
One method for improving catalytic converter performance during startup is by utilizing a secondary air injection (SAI) system. The output of an SAI pump is disposed in the exhaust system upstream of the catalytic converter and is used to inject air into the exhaust to react with unburned and partially burned fuel from the engine in order to heat up the catalyst. Because of SAI pump flow variation however, it is difficult to provide the optimal exhaust fuel to air ratio from one vehicle to another, over various engine operating and environmental conditions, and as the pump performance degrades with age. That is, the secondary air injected into the exhaust may not yield the optimal exhaust F/A entering the catalytic converter on a consistent basis.